Hack and / - Linux Troubleshooting, Part II: Local Network

Last month, I discussed localhost troubleshooting, and this month, I extend troubleshooting to your local network. Find out why shawn can't talk to bill.

This column is the second in a series dedicated to one of my
favorite subjects: troubleshooting. Because my column is generally aimed
more at tips and tricks and less on philosophy and design, I'm not going
to talk much about overall approaches to problem solving. Instead, in
this series, I describe some general classes of problems you
might find on a Linux system, and then I discuss how to use common
tools, most of which probably already are on your system, to isolate
and resolve each class of problem.

In the first column, I talked about how to diagnose high-load issues on a
server, but the fact is that these days, just about every Linux computer
is connected to a network, and a large number of the problems you have
are based in the network. This month, I focus on local
network troubleshooting, and although I am writing from the perspective
of servers, most of these steps will apply to any Linux machine on a
network. Also, because the goal of this article is to show how to become
better at troubleshooting, I list each step from the lowest
level on up. In real life, I'd probably skip ahead here and there to make
the troubleshooting process faster.

bill Is Down

The generic problem I cover here is how to track down the root
cause when one machine can't communicate with another machine on the same
network. For this example, let's assume I have two servers named bill and
shawn. The server shawn is trying to communicate with bill over port 25
(port 25 is used for sending e-mail over SMTP), but wouldn't you know it,
bill isn't responding.

Does shawn or bill Have a Problem?

One of the first things I might do in a scenario like this is find
another machine on the same network and try to connect with bill from
there. If I can talk to bill from another machine on the same network,
the problem is most likely with shawn or with the network in between
shawn and bill. If I have the same problem from another machine on the
same network, it's more likely that the problem is with bill, so
I would start troubleshooting from there. Just so I can discuss more
troubleshooting steps, let's start troubleshooting from shawn.

One of the most embarrassing things in troubleshooting is to waste
an hour only to find out that something wasn't plugged in. So the
first step I perform is to make sure that shawn is plugged
in to the network. Although I could inspect the port physically on the
server, if the server were in a different city, I might run a program
like ethtool. ethtool gives you a lot of different diagnostics on your
Ethernet devices. By default, all you have to do is run ethtool as root
and pass the Ethernet device you want to check as an argument. In many
cases this will be eth0:

As you can see, ethtool gives all sorts of information, including
the fact that this machine supports 10 base T, 100 base T and gigabit
networking speeds, but it currently communicates at 100 base T, full
duplex. To check for a link, just look at the very last line that says
“Link detected”. As you can see in my example, link is detected, so my
cable is plugged in and I can move on.

Before I move past ethtool completely, it's worth mentioning that it
does a lot more than just diagnose link problems. A common problem I've
found on networks is a host with slower-than-normal network speeds. Often
you'll see this crop up after a reboot or a power outage. What often
happens is that when the interface connects to the network, it will try
to auto-negotiate the fastest speed it can. Sometimes auto-negotiation
doesn't work correctly, in which case the interface might fail back to
half duplex mode or might even fail back to 10 base T! If you know that
your network can support 100 base T at full duplex, you can use ethtool
to disable auto-negotiation and force full duplex. To do this for eth0,
you would type:

Kyle Rankin is a VP of engineering operations at Final, Inc., the author of
a number of books including DevOps Troubleshooting and The Official Ubuntu
Server Book, and is a columnist for Linux Journal. Follow him @kylerankin.

Geek Guides

Pick up any e-commerce web or mobile app today, and you’ll be holding a mashup of interconnected applications and services from a variety of different providers. For instance, when you connect to Amazon’s e-commerce app, cookies, tags and pixels that are monitored by solutions like Exact Target, BazaarVoice, Bing, Shopzilla, Liveramp and Google Tag Manager track every action you take. You’re presented with special offers and coupons based on your viewing and buying patterns. If you find something you want for your birthday, a third party manages your wish list, which you can share through multiple social- media outlets or email to a friend. When you select something to buy, you find yourself presented with similar items as kind suggestions. And when you finally check out, you’re offered the ability to pay with promo codes, gifts cards, PayPal or a variety of credit cards.